Answers

Are you sure they've been eaten - many people have lost busy lizzies this year because the plants have just withered and died, for some reason. Don't know where you live, exactly, so don't know what other wildlife you might have about that finds busies delectable.

If the damage is to the roots, the culprit is probably Vine weevil larvae - white grubs with a brown head in the soil beneath the plant. (the roots will be gone). In small quantities best killed by squashing the larvae, they don't fly.